The present invention relates to a single-panel color projector which uses only a single light valve for light modulation.
The current mainstream of color projector is in three-panel projectors that use three light valves such as liquid crystal panels. This is because they are easy to install, have good portability, and provide high brightness. However, the light valve is costly. Thus, the overall cost of apparatus that uses three light valves is increased.
There is also known a single-panel color projector that uses only one light valve such as a liquid crystal panel. This type of color projector is lower in cost than those using three light valves. A conventional single-plate color projector, for example, one which uses a mosaic color filter for the light valve as disclosed in Japanese Unexamined Patent Publication No. 59-230383 has a problem that image brightness is lowered because about two-third of illumination light is absorbed or reflected by that color filter. Moreover, the resolution required with the single light valve is three times as high as that of the light valves in the three-plate color projector. Thus, the cost of the light valve will become very high and a considerable reduction in overall cost cannot therefore be expected.
A color projector as disclosed in Japanese Unexamined Patent Publication No. 4-60538 is arranged to separate three primary colors of light in angular directions by means of three dichroic mirrors and cause the three primary colors of light to focus onto their respective target areas by microlenses, thereby performing the function of a color filter. This significantly improves utilization of illumination light. However, in this type of color projector as well, as with the color projector using color filter, the resolution is required to be three times as high as the resolution of each of three light valves in the three-panel color projector and hence a considerable reduction in overall cost cannot be expected.
As another single-panel type of color projector, a color sequential display type of color projector is also known which uses a rotating color filer disc. In this color projector, one pixel is irradiated sequentially with red, green and blue colors of light to provide full color display. In this system, the resolution required with the light valve remains unchanged from that in the three-plate color projector. However, this system requires a light valve with a short response time. Therefore, liquid crystal panels as used in normal color projectors cannot be used because they have too long a response time. For this reason, light valves, such as DMDs (Digital Micromirror Devices) or ferroelectric liquid crystal panels, that have a short response time will be used. However, such light valves are bistable devices which are switched between on and off states. For example, the representation of gray scales by the valve is realized by the use of PWM (Pulse Width Modulation), i.e., by varying its on time. In this system, about two-third of illumination light is lost because of absorption or reflection by the color filter disk, which will result in a problem of low image brightness.
As a high-speed light valve, a reflective liquid crystal panel oriented toward micro-display (head-mounted display) has been developed which sequentially turns on red, green and blue LEDs to provide color sequential display. In application of this liquid crystal panel in a liquid crystal projector, red, green and blue light will be taken out of white light from a white light source by the use of a rotating color filter disk, as used in a single-panel color projector using DMD, and then projected sequentially onto the liquid crystal panel. This system needs a rapid-response liquid crystal light valve.
The responsiveness of the liquid crystal light valve depends on the response characteristic of a liquid crystal material and the response characteristic of switching devices. As rapid-response liquid crystal materials, ferroelectric liquid crystal materials and anti-ferroelectric liquid crystal materials have been developed. These materials perform a digital-like operation of blocking light or letting it pass and the representation of gray scales is carried out by controlling the length of time that light passes through. Therefore, these materials must be used in combination with very fast switching devices. Amorphous silicon as the material of switching devices widely used in transmissive liquid crystal light valves is low in electron mobility and is not therefore suitable for fast switching. On the other hand, crystalline silicon used in reflective liquid crystal valves is high in electron mobility and hence allows fast switching required with the color sequential system. This system allows full color display with one pixel of liquid crystal and can solve the problem of the conventional single-panel system that there is the need for a light valve whose resolution is at least three times higher than with the three-panel system in which three light valves are used. However, it is impossible to solve the other problem that only one-third of light from a light source is utilized because of the use of the rotating color filter disk and bright image display cannot therefore be provided.
Moreover, a color-sequential projector is also known which employs a rotating prism. For example, in U.S. Pat. No. 5,528,318 there is disclosed a system which decomposes white light from a white light source into red, green and blue color band sets and scans or moves these color band sets over a light valve using the rotating prism. This system allows the panel resolution to be the same as with the three-plate system. In addition, this system has the advantage of being high in image brightness. However, this system has the following problems: (1) Color nonuniformity occurs because the color band scanning speed is not constant with respect to the rotation of the prism, and (2), since the color band sets are narrow in width, the length of time that a band moves across a certain line of pixels is very short and hence the existing DMDs and ferroelectric liquid crystal panels cannot represent required gray scales within that length of time with the use of PWM control.
As described above, the conventional single-panel color projector cannot increase image brightness using an existing light valve and cannot realize the representation of sufficient gray scales at low cost.
It is therefore an object of the present invention to provide a single-panel color projector which permits image brightness to be increased, cost to be reduced and sufficient gray scales to be represented through the use of an existing light valve.